1. Field of the Invention
The present invention relates to a color image forming apparatus and, in particular, to a color image forming apparatus which has a plurality of pairs of optical scanning devices and corresponding image carriers, optical scanning being performed on each image carrier with a beam emitted from each optical scanning device to thereby form an image, or a color image forming apparatus which has an optical scanning device including a multi-semiconductor laser and a plurality of image carriers, optical scanning being performed on the image carriers with a plurality of beams emitted from the optical scanning device to thereby form an image. The image forming apparatus is suitably applicable, for example, to a high speed laser printer apparatus, a plural-drum type color copying machine or a digital color copying machine.
2. Description of the Related Art
Conventionally, in a color image forming apparatus such as a plural-drum type color printer or a plural-drum type color copying machine, a laser exposure device, that is, an optical scanning device has been used which provides a plurality of image forming sections corresponding to color components obtained through color separation with image data corresponding to the color components, that is, a plurality of beams.
Generally speaking, an optical scanning device comprises a semiconductor laser device serving as the light source, a first lens unit (collimator lens, cylindrical lens, etc.) for reducing the diameter of a beam emitted from the semiconductor laser device to a predetermined dimension, a light deflector for continuously deflecting and reflecting the beam whose diameter has been reduced by the first lens unit in a direction perpendicular to the direction in which the recording medium is conveyed, a second lens unit (f.theta. lens system) for forming an image at a predetermined position of the recording medium from the beam deflected and reflected by the light deflector, etc.
In known examples of such an optical scanning device, a plurality of optical scanning devices are arranged in correspondence with the image forming sections, or a multi-beam optical scanning device capable of providing a plurality of beams is arranged.
In an example of the multi-beam optical scanning device, lens units such as f.theta. lenses are arranged in a number corresponding to the number of beams, as disclosed in Japanese Patent Laid-Open No. 3-264970. Further, as disclosed in Japanese Patent Publication No. 2-20986, there is arranged in the vicinity of the photosensitive member (photosensitive drum) a cylindrical lens for causing a plurality of beams to impinge upon a single light deflector and to be condensed by a single f.theta. lens (scanning lens) to correct any bend of the scanning line.
However, the above conventional multi-beam optical scanning devices have the following problems.
In the device disclosed in Japanese Patent Publication No. 2-20986, the beam impinges upon the f.theta. lens obliquely, so that the scanning line is curved, and the curved scanning line is corrected by a cylindrical lens. However, the correction cannot be effected completely. When there are a plurality of incident beams, there exist a plurality of incident angles, and the scanning lines with different curvatures cause positional deviation on the surface of the photosensitive member.
In the device disclosed in Japanese Patent Laid-Open No. 3-264970, the number of lens units such as f.theta. lens are arranged to correspond to the number of beams. In the case of this arrangement, the plurality of beams pass the centers of the respective optical systems, so that the scanning line is not easily curved. However, unless the accuracy of the optical systems is improved to an extreme degree, positional deviation in the scanning direction is generated on the surface of the photosensitive member due to errors in production. To avoid this, the lens accuracy is improved to an extreme degree, or a lens position adjusting mechanism is provided, resulting in the apparatus as a whole being complicated. Further, this leads to an increase in production cost.